1) Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell using lithium nickel composite oxide as the positive electrode active material.
2) Description of the Related Art
In recent years, there has been rapid enhancement of functionality of mobile information terminals such as laptop computers, creating a need for cells of higher capacity.
Non-aqueous electrolyte secondary cells, for their high energy density and high capacity, are widely used as power sources for mobile appliances.
As the positive electrode active material for the non-aqueous electrolyte secondary cells, lithium cobaltate has been conventionally used. In recent years, cells using lithium nickel composite oxide instead of lithium cobaltate have been developed.
Use of lithium nickel composite oxide as the positive electrode active material enhances the discharge capacity as compared with use of lithium cobaltate.
The lithium nickel composite oxide used in the non-aqueous electrolyte secondary cells is synthesized by using alkali such as lithium hydroxide.
At the time of the synthesis, the alkali reacts with carbon dioxide gas existing in the surrounding atmosphere to generate lithium carbonate (Li2CO3). Since this lithium carbonate remains on the surface of the lithium nickel composite oxide, when the cell using the lithium nickel composite oxide is preserved at high temperature, the lithium nickel composite oxide is decomposed to generate carbon dioxide gas.
Thus, the cells using lithium nickel composite oxide have the problem of cell swelling.
Examples of the prior art techniques related to non-aqueous electrolyte secondary cells include Japanese Patent Application Publication Nos. 2002-117843 (patent document 1), 2005-322616 (patent document 2), 2005-50707 (patent document 3), 4-329268 (patent document 4), 10-214640 (patent document 5), 6-196199 (patent document 6), and 7-245105 (patent document 7).
Patent document 1 discloses: use of a compound having an olivine structure as the positive electrode active material; and containing lithium carbonate in the positive electrode active material layer. This technique is claimed to provide a cell excellent in high-temperature cycle characteristic.
This technique, however, has such a problem that the lithium carbonate is decomposed to generate carbon dioxide gas, resulting in cell swelling on a large scale.
Patent document 2 discloses use of a positive electrode active material containing lithium nickel composite oxide and a layer held on the surface of the lithium nickel composite oxide and composed of lithium carbonate, aluminum hydroxide, and aluminum oxide. This technique is claimed to provide a cell excellent in initial capacity and life characteristic.
This technique, however, has such a problem that the lithium carbonate is decomposed to generate carbon dioxide gas, resulting in cell swelling on a large scale.
Patent document 3 discloses dissolving cyclic acid anhydride and carbon dioxide gas in the non-aqueous electrolytic solution. This technique is claimed to provide a cell excellent in cycle characteristic and rapid charging characteristic.
This technique, however, still cannot provide sufficient cycle characteristic.
Patent document 4 discloses use of, as the positive electrode active material, LixMO2 (M being at least one selected from Co and Ni) covered on its surface with lithium carbonate. This technique is claimed to inhibit rapid increase in temperature and rapid damage.
This technique, however, has such a problem that the lithium carbonate is decomposed to generate carbon dioxide gas, resulting in cell swelling on a large scale.
Patent document 5 discloses providing a free acid adsorbing layer between the positive electrode active material and the separator. This technique is claimed to provide a cell excellent in cycle characteristic.
This technique, however, still cannot provide sufficient cycle characteristic.
Patent document 6 discloses providing, between the positive electrode and the negative electrode, a multi-layer metal oxide film made of a bilayer formed into a mold shape. This technique is claimed to provide a cell excellent in cycle characteristic.
This technique, however, still cannot provide sufficient cycle characteristic.
Patent document 7 discloses use of a positive electrode active material wherein the surface of lithium nickelate is partially or entirely covered with lithium carbonate. This technique is claimed to provide a cell excellent in high-temperature preservation characteristic.
This technique, however, has such a problem that the lithium carbonate is decomposed to generate carbon dioxide gas, resulting in cell swelling on a large scale.